This invention relates to a rotation transmission device for transmitting driving force in a four-wheel drive vehicle, and more specifically a device mounted in the drive train of a four-wheel drive vehicle for selectively transmitting the driving force.
When a four-wheel drive vehicle (hereinafter 4WD) turns a tight corner with the four wheels coupled together on a paved road, the vehicle may experience what is known as "tight corner braking effect". To solve this problem, the applicant of this invention proposed a rotation transmission device having sprags or rollers as engaging elements in Japanese patent publications 6-17853 and 6-33957.
These devices are two-way driving units having a retainer moveable to one of two positions according to the direction of rotation. In either of the two positions, the engaging elements engage the inner and outer rings. Either of them may be mounted on the front wheel propeller shaft of an FR-based 4WD as shown in FIG. 25. If one of the rear wheels slip while the vehicle accelerates, the rotating speed of the input shaft will increase, so that the engaging elements will engage. The drive mode thus changes to four-wheel drive position. While the vehicle beings turning a corner, the front wheels (output shaft) begin rotating faster than the rear wheels. This disengages the engaging elements of the rotation transmission device, allowing freewheeling of the input shaft and the output shaft. Thus, no tight corner braking effect will occur.
During engine braking, the engine force decelerates the input shaft of the rotation transmission device, so that the input shaft and the output shaft are disconnected from each other. This means that the engine braking force is not transmitted to the front wheels. That is, the engine brake is applied only to the rear wheels. Thus, if strong engine brake is applied while the vehicle is traveling on a low-friction-coefficient road such as a snow-covered road, the rear wheels may lock up and lose grip of the road surface, thus destabilizing the vehicle. In an extreme case, the vehicle may spin.
Also, suppose the 4WD equipped with the device proposed in either of the above publications is further provided with an ABS (antilock braking system). Generally speaking, stronger braking force is distributed to the front wheels than to the rear wheels, so that the front wheels tend to lock up before the rear wheels.
When the front wheels begin to lock, the output shaft (connected to the front wheels) are decelerated quickly, so that the engaging elements engage. The input shaft is thus decelerated together with the output shaft, so that the rear wheels are locked simultaneously with the front wheels. Since all the wheels are locked, the ABS cannot detect that the wheels are locked because it cannot detect speed differences between the front wheels and the rear wheels.
An object of this invention is to provide a rotation transmission device which transmits driving force for rotation in either direction, which can prevent tight corner braking torque to the front wheels as well as to the rear wheels, and which can idle in either direction while the ABS is on.
Another object is to provide a method for controlling such a rotation transmission device.